Catalytic cracking is an established and widely used process in the petroleum refining industry for converting oils and residua of relatively high boiling point to more valuable lower boiling products including gasoline and middle distillates such as kerosene, jet fuel and heating oil. The preeminent cracking process now in use is the Fluid Catalytic Process (FCC) in which the preheated feed is brought into contact with a hot cracking catalyst that is in the form of a fine powder, typically with a particle size of 10-300 microns, usually about 60 microns, for the desired cracking reactions to take place. During the cracking, coke is deposited on the catalyst and this results in a loss of activity and selectivity. The coke is removed by continuously removing the deactivated catalyst from the cracking reactor and oxidatively regenerating it by contacting it with air in a regenerator. The combustion of the coke is a strongly exothermic reaction which, besides removing the coke, serves to heat the catalyst to the temperatures appropriate for the endothermic cracking reaction. The process is carried out in an integrated unit comprising the cracking reactor, the regenerator and the appropriate ancillary equipment. The catalyst is continuously circulated from the reactor to regenerator and back to the reactor with the circulation rate being adjusted relative to the feed rate of the oil to maintain a heat balanced operation in which the heat produced in the regenerator is sufficient to maintain the cracking, with the circulating, regenerated catalyst being used as the heat transfer medium. Typical fluid catalytic cracking processes are described in the monograph Fluid Catalytic Cracking with Zeolite Catalysts, Venuto, P. B. and Habib, E. T., Marcel Dekker Inc., N.Y. 1979, to which reference is made for a description of such processes. As described in the monograph, the catalysts which are currently used are based on zeolites, especially the large pore synthetic faujasites, Zeolites X and Y, which have generally replaced the less active, less selective amorphous and clay catalysts formerly used.
Another catalytic cracking process still used in the industry is the moving, gravitating bed process, one form of which is known as Thermofor Catalytic Cracking (TCC) which operates in a similar manner to FCC but with a downwardly moving gravitating bed of a bead type catalyst, typically about 3-10 mm in diameter. Fixed bed units have now been replaced by fluidized or moving bed units of the FCC or TCC type. It should be noted that all references made herein to "catalytic cracking", "fluid catalytic cracking", FCC and TCC processes, and the like, refer to and mean cracking in the absence of added hydrogen, as distinguished from hydrocracking, in which the feed and gaseous hydrogen are contacted with a hydrocracking catalyst. Although both processes result in boiling point reduction of a heavy oil, the catalyst, the process conditions, the process itself, and the nature of the products are very different.
The feed to the catalytic cracker can generally be characterized as a high boiling oil or residuum, either on its own or mixed with other fractions, usually of a high boiling point. The most common feeds are gas oils, that is, high boiling, non-residual petroleum distillate oils with an initial boiling point usually above about 230.degree. C. (about 450.degree. F.), more commonly above about 345.degree. C. (about 650.degree. F.), with end points of up to about 620.degree. C. (about 1150.degree. F.). Typical gas oil feeds include straight run (atmospheric) gas oil, vacuum gas oil and coker gas oil; residual feeds include atmospheric residua, vacuum residua and residual fractions from other refining processes. Oils from synthetic sources such as Fischer-Tropsch synthesis, coal liquefaction, shale oil or other synthetic processes may also yield high boiling fractions which may be catalytically cracked either on their own or in admixture with oils of petroleum origin.
Aside from foreign matter such as rust, brine, sand and water, catalytic cracker feeds consist almost totally of organic compounds, some of which may be organometallic. More directly pertinent to the present invention is that the organic compounds which make up the feed, i.e. the very complex mixture of paraffins, naphthenes and aromatic compounds, normally include a small but significant complement of sulfur, nitrogen and oxygen heteroatoms. These elements often are referred to as "contaminants" in petroleum refining technology because they serve no obvious useful role in fuel and lubricant products, but contribute instead to catalyst deterioration in processing, and to air pollution when the product is used as a fuel. It is well to recognize, however, that these heteroatom contaminants are integral parts of the chemical structure of the hydrocarbon feed, i.e. they are chemically bound parts of the feed molecules in the same sense as is the nitrogen atom in pyridine. The term "organic nitrogen" as used herein means contaminant nitrogen that is a chemically bound part of the organic feed or of the coke.
In view of their nature, it becomes understandable that the heteroatom contaminants in the feed cannot be readily removed by conventional means which are effective with physical contaminants such as sand and rust. The organic nitrogen in the feed is very pertinent to the present invention since it is generally conceded to be the only significant precursor for the noxious nitrogen compounds in the regenerator flue gas. Feeds in general contain from 0.05 to 0.5 weight percent nitrogen (500-5000 ppmw) although some synthetic feeds such as shale oil may have higher contents.
In the catalytic cracker, the feed together with its organic nitrogen are raised to a cracking temperature usually in the range of about 470.degree. to 520.degree. C. (riser top temperature) by contact with hot regenerated catalyst, under which conditions a portion of the feed is cracked, with simultaneous formation on the catalyst of a carbonaceous deposit which is generally called "coke". This coke deposit is very largely formed of carbon and hydrogen, and usually it can be described by the empirical formula of C.sub.n H.sub.0.5 n - C.sub.n H.sub.n. More careful examination of the coke shows that it contains a small amount of organic nitrogen, generally somewhat less than can be accounted for by the total nitrogen in the feed.
The coked catalyst, on being passed to the regenerator, is contacted with air at a temperature usually of about 650.degree. to 750.degree. C. to provide hot regenerated catalyst which is returned to the cracker, and a flue gas. Almost all of the flue gas consists of elemental nitrogen introduced with the air, together with relatively large amounts of the expected combustion products, including water vapor, carbon monoxide and carbon dioxide. Of these combustion products, the water vapor is inherently benign, and the carbon dioxide, although of long-range ecological interest, has low toxicity and may be regarded as a necessary waste product of catalytic cracking, barring some unforeseen and radical change in future cracking technology. Carbon monoxide, however, is a different matter. Carbon monoxide is produced together with carbon dioxide whenever carbonaceous material is burned. When burning FCC. coke, the ratio of CO.sub.2 to CO in the flue gas is known to depend on combustion conditions including temperature, availability of oxygen, and the presence or absence of CO-combustion catalysts. Unlike carbon dioxide, carbon monoxide is toxic and it is known to contribute to urban smog. Also, unlike carbon dioxide, carbon monoxide has value as a fuel and may be used to produce steam. In the absence of a CO-combustion catalyst, the ratio of CO.sub.2 /CO in the flue gas usually is in the range of about 1.0 to 2.0. Since two thirds of the heat of combustion of carbon to CO.sub.2 is associated with the conversion of CO to CO.sub.2, it is apparent that such conversion can generate significant heat value. While the negative attributes of carbon monoxide led government authorities to impose restrictions on allowable emissions of this substance, its fuel value suggested the use of a CO-boiler as a low-cost means of compliance. Thus, for at least several decades before 1972 when a new FCC. catalyst became available, refiners controlled burning in the regenerators of FCC. plants to form a flue gas relatively rich in CO but limited in excess oxygen, and passed this flue gas with additional air to a CO-boiler to recover heat values as process steam.
The primary purpose of a refinery CO-boiler is to incinerate the flue gas formed in the FCC. regenerator. In the CO-boiler, the flue gas is incinerated by mixing with a high excess air flame from a conventional burner operated typically on refinery gas fuel. The fuel gas burner supplies both heat and oxygen for oxidation of virtually all of the CO to carbon dioxide (CO.sub.2) if the unit is properly designed for rapid and complete mixing of the burner products with the flue gas. This mixing must occur early in the furnace well before the gases contact convective cooling surface. In most cases, the unit must be designed for effective mixing over a range of fuel/flue gas input ratios since the boiler may follow refinery stream demand by variation in fuel input rate while flue flow rate remains essentially constant.
In a paper presented at the 77th Annual Meeting of the Air Pollution Control Association, Jun. 24-29, 1984, H. B. Lange et al. present a study on NO.sub.x emissions from CO-boilers, including some useful bench mark reactions which are here reproduced. EQU N.sub.2 +O.sub.2 .fwdarw.NO (1)
They report that this reaction takes place in the absence of a catalyst and in the presence of excess oxygen at temperatures above about 1525.degree. C. in the boiler furnace, and is avoided by adjustments to reduce simultaneous occurrence of high O.sub.2 and high temperature levels. The next reaction discussed is: EQU NH.sub.3 +O.sub.2 .fwdarw.NO+H.sub.2 O (2)
This oxidation takes place in the absence of a catalyst and in the presence of excess oxygen at temperatures about 1025.degree. C.
Denitrification reactions include: EQU NH.sub.3 +NO (fuel rich mix).fwdarw.N.sub.2 +H.sub.2 O (3) EQU NH.sub.3 +NO (excess O.sub.2).fwdarw.N.sub.2 +H.sub.2 O, (4)
Reaction 3 is reported to proceed in the absence of a catalyst in the temperature range of about 1100.degree. to 1210.degree. C., and proceeds only in a fuel-rich mixture. Reaction 4 is reported to proceed in the somewhat lower temperature range of about 925.degree. to 1025.degree. C. Equations 1-4 represent complex reaction chains and show only major species; they are not stoichiometrically balanced.
In about 1972, a catalyst modification became available which allowed the refiner to burn up part or all of the carbon monoxide in the regenerator, thus providing emissions control of carbon monoxide and recovery of heat values without use of a CO-boiler. This development is described in U.S. Pat. Nos.: 4,072,600; 4,088,568; and 4,093,533, all to Schwartz, incorporated herein by reference for background purposes.
In addition to the major combustion products described above, flue gas also contains very much smaller quantities of nitrogen oxides. Although several nitrogen oxides are known which are relatively stable at ambient conditions, it is generally recognized that two of these, viz. nitric oxide (NO) and nitrogen dioxide (NO.sub.2), are the principal contributors to smog and other undesirable environmental effects when they are discharged into the atmosphere. These effects will not be discussed further here since they are well recognized and have led various government authorities to restrict industrial emissions in an attempt to limit the level of the oxides in the atmosphere. Nitric oxide and nitrogen dioxide, under appropriate conditions, are interconvertible according to the equation EQU 2NO+1/2O.sub.2 .revreaction.2 NO.sub.2 (5)
For purposes of the present invention, NO.sub.x will be used herein to represent nitric oxide, nitrogen dioxide, and mixtures thereof.
The so-called "stable" nitrogen oxides have in common the somewhat peculiar property that although they are thermodynamically very unstable with respect to decomposition into elemental oxygen and nitrogen, no simple, economical method has been described for inducing this decomposition. It has been discovered, however, that adding a reductant such as ammonia to the exhaust gas can, under appropriate reaction conditions, convert NO.sub.x to elemental nitrogen and steam.
U.S. Pat. No. 3,900,554 to Lyon describes a homogenous gas phase reaction to remove NO.sub.x from combustion effluents by adding 0.4 to 10 mols (preferably 0.5 to 1.5 mols) of ammonia per mole of NO.sub.x followed by heating to 1600.degree. C. to 2000.degree. C. (See Equation 4, above.) The NO.sub.x content is lowered as a result of its being reduced to nitrogen by reaction with ammonia. The method is reported to work best if hydrocarbon is also added to the mixture.
U.S. Pat. No. 4,220,632 to Pence et al. discloses a process for reducing noxious nitrogen oxides from a fossil-fuel-fired power generation plant, or from other industrial plant off-gas stream, to elemental nitrogen and/or innocuous nitrogen oxides employing ammonia as reductant and, as catalyst, the hydrogen or sodium form of a zeolite having pore openings of about 3 to 10 Angstroms. The process of adding ammonia to industrial flue gas followed by contact with a catalyst at a temperature in the range of about 250.degree.-550.degree. C. to denitrify the flue gas has come to be known as the process for Selective Catalytic Reduction (SCR) of NO.sub.x. In order to avoid confusion, any reference made herein to "Selective Catalytic Reduction", or to "SCR", is intended to refer to only that process in which a mixture of NO.sub.x and NH.sub.3 are induced to react catalytically at elevated temperature, and to exclude processes in which other reductants such as CO or hydrogen gas are substituted for NH.sub.3.
The term "denitrify" as used herein, means to reduce the amount of one or more noxious nitrogen compounds (such as NO, NO.sub.x and HCN) contained in a waste gas, preferably by conversion to nitrogen gas, or else to a relatively innocuous nitrogen compound such as nitrous oxide (N.sub.2 O). (See, for example, "Webster's New World Dictionary" 2nd College Edition, Prentice Hall Press, 1984.)
U.S. Pat. No. 4,778,665 to Kiliany et al. describes an SCR process for pretreating industrial exhaust gases contaminated with NO.sub.x in which the catalyst has a silica to alumina ratio of at least 50 and a Constraint Index of 1 to 12. The entire contents of this patent are incorporated herein by reference as if fully set forth.
While the SCR process does furnish a means for abatement of NO.sub.x emissions, it is relatively complex, it requires furnishing large amounts of ammonia, and it requires a sophisticated control system to insure that neither excess amounts of NO.sub.x nor unreacted ammonia is emitted. There is clear need for novel, inexpensive methods for abatement of industrial NO.sub.x emissions.
Before describing the role of hydrogen cyanide (HCN) in denitrification of flue gases, a brief description of its relevant properties is in order. HCN is a liquid at atmospheric pressure that boils at 25.7.degree. C. and freezes at -13.24.degree. C. It is highly flammable, and burns with a heat of combustion of 159.4 kCal/mol. It forms explosive mixtures with air, and it has unlimited solubility in water. For further description of its physical and chemical properties, including health and safety factors in handling, see "Encyclopedia of Chemical Technology", Kirk-Othmer, Vol. 7, pp. 307-319 (1979), incorporated herein by reference for background.
HCN is hydrolyzed in the presence of an appropriate catalyst to form ammonia by the reaction: EQU HCN+H.sub.2 O=NH.sub.3 +CO (6)
European Patent 0051156 describes the conversion of HCN to NH.sub.3 over copper oxide, iron oxide, or chromium oxide (Cr.sub.2 O.sub.3) at temperatures of between 100.degree. and 500.degree. C.; conversions of more than 90% are disclosed. A process is described in Japanese Disclosure 53005065 in which HCN is hydrolyzed to ammonia by at least one oxide of the elements Al, Ce, Zr, Mg, Ca, Ba, Na, K, Mo, V, Fe, Co, Ni, Cu, Mn, Ag, and La. Patent DE-OS 23 41 650 describes a process in which HCN is hydrolyzed with H.sub.2 O at temperatures above 93.degree. C. in the presence of a catalyst that contains at least an alkali metal hydroxide supported on aluminum oxide, silica, silica-alumina, or a zeolite. The patents noted above are incorporated herein by reference as if fully set forth.